Some of the superconducting oxides found recently have a superconducting critical temperature (Tc) higher than the boiling point of liquid nitrogen so that the applications of superconductors have been broadened significantly. Getting a superconducting device expected to be applied in an electronics field, some kinds of superconducting junctions are produced on a trial basis as a basic element of the device. The junctions are, for example, those which utilize in-plain grain boundaries, those which utilize the differences of surfaces like steps and those of a multilayer-type (S:Takada, Oyo Buturi, vol.62, 1993, p443).
The superconducting junctions which have been produced so far, however, have not provided a satisfactory effect. In other words, tunnel junction elements showing an ideal Josephson characteristics have been hard to obtain. This is because it is technically hard to completely get rid of disorder from tunnel junctions due to the contacts between superconducting compounds and non-superconduting compounds especially when including a composite oxide showing a high Tc. Though it was reported that a single crystal of Bi-based superconducting oxide showed characteristics like tunnel junction ( K. Tanabe et al, Physical Review B, vol.53, No.14, April 1996), the characteristics were observed only in the single crystal so that it was hard to apply it to a useful element or device in an electronics field.
As described above, it has been hard to produce a tunnel junction element which can be applied to a superconduting device. In order to get a good contact between a superconduting layer and a non-superconducting layer and make the element practical, it is necessary that the compounds of the layers are compatible and the element is not hard to produce. A tunnel junction element including a niobium (Nb) layer known as a superconducting metal, was not actually realized until a superb combination of the metal as electrodes and an aluminium oxide as a barrier layer was discovered. Such a superb combination including a superconducting oxide has been hard to produce, to date.